The present invention relates to the field of fiber optic data communications, in particular fault detection arrangements in a DC coupled parallel optical link.
Laser-based devices and systems have been used widely in the fields of, for example, communications, computing technology and medical technology. The lasers utilized in these systems have output optical powers that are potentially harmful to both people and equipment. For instance, such lasers are driven at such a power so as to have damaging effects if exposed to a human eye. Among the safety methods and systems that have been developed, Method and Apparatus for Laser Safety described in U.S. Pat. No. 5,999,549 (Freitag, et al.), resets a laser fault counter if a second laser fault condition is not detected within a predetermined reset time period after a laser is turned on.
In the field of fiber optic data communications, fiber optic data communications links must ensure the optical power being transmitted by the laser remains below a defined level, or a xe2x80x9csafexe2x80x9d level, in the event of a single failure in the link so as to avoid the aforementioned potential harm to both people and equipment. The xe2x80x9csafexe2x80x9d level may include, for example, a standard established by industry and/or governmental regulations.
For serial optical links, there are at least two exemplary methods for ensuring that transmitted, optical power does not exceed the relevant xe2x80x9csafexe2x80x9d level, thus ensuring the safety of the users and any surrounding people as well as preventing any damage to the apparatus by the laser optical power. A first example method includes setting the optical power delivered by the laser to a level that is well below the xe2x80x9csafexe2x80x9d level and utilize circuits on the transmitter IC to detect when the optical power level exceeds the safe level. Since the optical power in serial optical links is most often controlled by a monitor photodiode control loop, the laser average optical power is known. Therefore, fault detection circuits are able to easily determine when the average optical power exceeds a threshold limit. That is, since the current in the monitor photodiode is proportional to the optical power output by the laser, the transmitter can detect when the monitor photodiode current exceeds a threshold limit.
A second example method for ensuring that transmitted optical power does not exceed the safe level in a serial optical link includes an Open Fiber Control (OFC) handshake protocol. This example protocol is used when the laser-driving optical power in normal data mode is set above the safe level. Thus, when a serial optical link fiber is pulled, according to the OFC protocol, the laser light is pulsed at an extremely low duty cycle (on for approximately 150 psec, off for approximately 10 sec) to ensure that the average laser optical power does not exceed the safe level. Similar to the first method, fault detection circuits on the transmitter side also ensure that a fault in the corresponding laser driving circuit does not cause the average optical power to exceed the safe level.
However, such example methods of ensuring that the laser optical power remains at or below a safe level are not applicable to open loop parallel optical links or even DC coupled open loop parallel optical links. That is, in open loop parallel optical links, including DC coupled open loop parallel optical links, the average optical power is unknown, because there are no monitor photodiodes. Also, the total average optical power of a parallel link can be greater than a serial link, because multiple laser are simultaneously emitting light. Thus, the example fault detection methods described above are inappropriate since the aggregate optical power in an open loop parallel optical link is above the safe level and is much higher than that of a serial link.
For instance, FIG. 11 shows an open loop parallel optical transmitter which includes N channels. The parallel optical transmitter 200 includes global temperature coefficient adjustment DAC (TEMPCO DAC) 210 and global temperature coefficient adjustment shift register (TEMPCO SHIFT REGISTER) 220 which holds bits for the TEMPCO DAC 210. Each of channels 0 through N include a respective laser driver 230, a threshold current adjustment DAC 240, a modulation current adjustment DAC 250, and a shift register to hold the bits for each DAC 260. EEPROM 270 stores the bits in a non-volatile memory when the parallel transmitter is powered off. This parallel transmitter, however, does not show any method for preventing the aggregate optical power out of the lasers of channels 0 through N from exceeding the xe2x80x9csafexe2x80x9d level.
Open loop parallel optical links, including DC coupled open loop parallel optical links, present a challenge, because the average optical power is not monitored by a photodiode and multiple lasers are emitting light simultaneously. The fault detection methods, therefore, are very much different from a serial link.
Also, DC coupled parallel optical links have inherently lower sensitivity than AC coupled links. A diagram of a single channel DC coupled receiver is shown in FIG. 9. In AC coupled optical links, as in the examples described above, the receiver""s decision threshold (difference between a logic 0 and a logic 1) is set by a capacitor. Capacitors are passive, lossless, and noiseless. Without capacitors, an active transistor network must be employed to set the decision threshold in the post amp 935 of FIG. 9. Transistors induce noise into the system, thus, reducing the signal to noise (S/N) ratio. A DC coupled link also causes problems with biasing the photodiode 920 using a low power supply voltage. The photodiode 920 cannot be biased independent of the transimpedance amplifier (TIA) 930. Instead, the feedback resistor 925 (Rf), must be used to bias the photodiode 920. With a low power supply voltage, the voltage drop across Rf 925 must be small, therefore, Rf 925 must be small. Since the noise induced by TIA 930 is inversely proportional to the value of Rf 925, reducing Rf 925 increases the noise of the TIA output. This, in turn, decreases the S/N ratio. Both the lower S/N ratio of the TIA 930 and the post amp 935 contribute to the lower sensitivity of the overall DC coupled parallel optical link. Compared to an AC coupled link, this leaves less difference between the safety limit and the minimum optical power that can be detected by the receiver. This phenomena is depicted in FIG. 10, wherein the safety limit is the same for both the AC and DC coupled links, although the minimum detectable optical power is lower for the AC coupled link. The low sensitivity of the DC coupled optical link requires higher optical power to be transmitted, which presents a safety problem.
It is, therefore, a principle object of this invention to provide a laser safety method for DC coupled parallel optical links.
It is another object of the invention to provide a laser safety method for DC coupled parallel optical links that solves the problems described above.
These and other objects of the present invention are accomplished by the laser safety method for DC coupled parallel optical links disclosed herein.
These and other objects are addressed by the present invention which includes a duplex DC coupled parallel optical link, by which the transmitted optical power is high enough to be detected by a receiver yet low enough so as not to not exceed a safety limit.
The present invention relates to a duplex DC coupled parallel optical link that includes a matched transmitter and receiver pair that cannot be physically detached from one another and a fiber optic ribbon cable that cannot be split. The duplex DC coupled parallel optical link of the present invention requires no OFC, thus greatly reducing the time between power up and when the link is ready to send data. There is also provided a margin between the peak optical power set point and the minimum sensitivity of the receiver by not requiring the peak optical power to be set below the safety limit.